Table Of ContentOECDDOCUMENTS
Reviewo f Nuclear Fuel
ExpevivMeMtDaa\ ta
FuelB ehaviouDr ata Availablefr om
IFE-OCDE HaldenP roject
for Developmenatn dV alidationo f ComputeCr odes
prepare6d g
JameAs nthonyT urn6ull
January 1995
PUBLISHER’S NOTE
The following texts are published in an unembelbshed form to pernut faster distnbutmn at a lower cost.
The views expressed are those of the authors.
and do not necessarily reflect those of the Organisation or of its Member countries.
ORGANISATION FOR ECONOMIC CO-OPERATION
AND DEVELOPMENT
Pursuant to Article 1 of the Convention signed in Paris on 14th December 1960, and which
came into force on 30th September 1961, the Organisation for Economic Co-operation and
Development (OECD) shall promote policies designed:
- to achieve the highest sustainable economic growth and employment and a rising
standard of living in Member countries, while maintaining financial stability, and thus to
contribute to the development of the world economy;
- to contribute to sound economic expansion in Member as well as non-member countries
in the process of economic development; and
- to contribute to the expansion of world trade on a multilateral, non-discriminatory basis
in accordance with international obligations.
The original Member countries of the OECD are Austria, BelgiumCanada, Denmark, France,
Germany, Greece, Iceland, Ireland, Italy, Luxembourg, the Netherlands, Norway, Portugal, Spain,
Sweden, Switzerland, Turkey, the United Kingdom and the United States. The following countries
became Members subsequently through accession at the dates indicated hereafter: Japan (28th April
1964) Finland (28th January 1969), Australia (7th June 1971) New Zealand (29th May 1973) and
Mexico (18th May 1994). The Commission of the European Communities takes part in the work of
the OECD (Article 13 of the OECD Convention).
NUCLEARENERGYAGENCY
The OECD Nuclear Energy Agency (NEA) was established on 1st February 1958 under the name
of the OEEC European Nuclear Energy Agency. It received its present designation on 20th April 1972,
when Japan became its first non-European full Member. NEA membership today consists of all European
Member countries of OECD as well as Australia, Canada, Japan, Republic of Korea, Mexico and
the United States. The Commission of the European Communities takes part in the work of the Agency.
The primary objective of NEA is to promote co-operation among the governments of its
participating countries in furthering the development of nuclear power as a safe, environmentally
acceptable and economic energy source.
This is achieved by:
- encouraging harmonization of national regulatory policies and practices, with particular
reference to the safety of nuclear installations, protection of man against ionising radiation
and preservation of the environment, radioactive waste management, and nuclear third party
liability and insurance;
- assessing the contribution of nuclear power to the overall energy supply by keeping under
review the technical and economic aspects of nuclear power growth and forecasting demand
and supply for the dtflerent phases of the nuclear fuel cycle;
- developing exchanges of scientific and technical information particularly through
participation in common services;
- setting up international research and development programmes and joint undertakings.
In these and related tasks, NEA works in close collaboration with the International Atomic Energy
Agency in Vienna, with which it has concluded a Co-operation Agreement, as well as with other
international organisations in the nuclear field.
0 OECD 1995
Applications for permission to reproduce or translate all or part
of this publication should be made to:
Head of Publications Service, OECD
2, rue AndrC-Pascal, 75775 PARIS CEDEX 16, France.
Credit of cover picture: Stress Corrosion Cracking in Zircaloy-4 (photo: CEA - Grenoble)
FOREWORD
This review has been carried out in the framework of activities of the OECD/NEA Task Force on
Scientific Issues in Fuel Behaviour. It is published on the responsibility of the Secretary-General of the
OECD. The opinions it expresses are those of the author only and do not represent the position of any
Member country or international organisation.
3
SUMMARY
This report outlines some of the experiments that have been performed in the Halden reactor and
the most important data they provided; these are available for use in fuel performance studies. It
discusses in addition some experiments from the Risø fission gas release projects and the Studsvik
Nuclear Ramp Programme. The report concentrates on experiments on thermal performance, fission
product release, clad properties and pellet-clad mechanical interaction. Experiments where integral
behaviour is measured using different types of instrumentation either within the same rod or the same
irradiation assembly are highlighted. These are of particular use in code validation. As a separate
topic, the report identifies experimental data relevant to high burnup behaviour.
The report recommends the preparation of an internationally available database covering as
many reactor systems as possible. This would greatly assist the development of codes and
mechanistic models which are preferable to those based on more restricted and specific data, as they
provide more reliable predictions of fuel performance. The report also suggests that the database be
structured around the topics discussed in the report. A priority list of experiments has been identified
for inclusion.
Finally, the review has determined that, despite the comprehensive nature of Halden data, further
experiments are required to extend the database to higher burnups, and that further measurements
are necessary to investigate the time dependence of fuel temperatures during rapid over-power
transients.
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CONTENTS
CHAPTER 1. INTRODUCTION................................................................................................................. 7
The Need for Data......................................................................................................... 7
CHAPTER 2. THE HALDEN REACTOR PROJECT.................................................................................... 9
The Experimental Programme...................................................................................... 9
CHAPTER 3. DATA REQUIREMENTS...................................................................................................... 11
CHAPTER 4. EXPERIMENTS AND DATA AVAILABLE............................................................................... 13
4.1 Radial Flux Depression......................................................................................... 13
4.2 Thermal Performance............................................................................................ 14
4.2.1 Beginning-of-Life Behaviour....................................................................... 14
4.2.2 Through-Life Behaviour.............................................................................. 15
4.3 Fuel Densification and Swelling............................................................................ 17
4.4 UO Grain Growth under Irradiation...................................................................... 18
2
4.5 Fission Product Release....................................................................................... 19
4.5.1 Radioactive Fission Products..................................................................... 19
4.5.2 Stable Fission Gas Release....................................................................... 19
4.6 Clad properties...................................................................................................... 21
4.6.1 Irradiation Growth....................................................................................... 21
4.6.2 Creep........................................................................................................... 21
4.6.3 Stress Corrosion Cracking.......................................................................... 22
4.6.4 Waterside Corrosion................................................................................... 22
4.7 Pellet-Clad Mechanical Interaction....................................................................... 23
4.7.1 Axial Elongation.......................................................................................... 23
4.7.2 Diametral Strain.......................................................................................... 24
4.7.3 Ridge Formation......................................................................................... 25
4.8 Integral Behaviour.................................................................................................. 26
4.9 High Burnup Effects .............................................................................................. 28
CHAPTER 5. FUTURE PROGRAMME...................................................................................................... 29
5.1 Data Available from the Halden Project................................................................ 29
5.2 Extension to Include Data from Other Sources.................................................... 30
CHAPTER 6. CONCLUSION AND RECOMMENDATION............................................................................. 33
ACKNOWLEDGEMENTS........................................................................................................................... 34
REFERENCES........................................................................................................................................ 34
TABLES................................................................................................................................................. 39
FIGURES............................................................................................................................................... 58
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CHAPTER 1
INTRODUCTION
In 1993, The NEA Nuclear Science Committee (NSC) set up a Task Force on Scientific Issues of
Fuel Behaviour. During discussions at the first meeting in December 1993 it was found that there was
a need for a better understanding of the underlying basic phenomena. Members were charged with
the task of identifying areas of high priority which would benefit from international co-operation and to
prepare a report providing advice on the developments needed in the form of experiments, data and
models, to meet the requirements for a better understanding of fuel behaviour.
One of the high priority actions identified and later approved by the NSC was the setting up of an
international database of well characterised experiments from different sources as a support for fuel
model development and validation. As a first step it was agreed to prepare a report reviewing existing
data sets and to select a subset that comprehensively covered the most important phenomena. This is
the subject of this report which reviews the data available from the OECD Halden Reactor Project and
touches on data available from other sources. The report also discusses the setting up of the
database and the issues to be addressed in doing so.
The Need for Data
Whereas in the past, reactor operation was supported by rather simple ‘scoping’ calculations to
ensure safety, today the requirements are much more onerous. Early bounding conditions were
generous compared to normal operation, but now, with increasing emphasis on economic as well as
safe operation, the approach to fuel performance is very much more detailed and closer to ‘best
estimate’ evaluation. Not only are the safety requirements more extensive, but the economics of
nuclear power production are constantly under scrutiny to ensure minimum unit cost. This is
particularly the case in countries where nuclear must compete with conventional stations. For these
reasons it is no longer possible to support operation with anything other than code calculations where
all aspects of fuel performance are treated simultaneously and in a self consistent way. With the
improvement of computer systems both in cost and speed, it is much more economic to have a small
number of dedicated codes and methodologies for performance assessment than a selection of labour
intensive stand alone calculations. Also, there is a move in some countries to ‘shadow’ operations
with an on-line evaluation of fuel performance and a continuous assessment of core conditions against
fixed limits for a large fraction of fuel assemblies in the reactor.
Thus the function of a fuel performance code is that it describes the behaviour of reactor fuel in
the most accurate way possible under whatever conditions both normal and off-normal that are
required by the licensing authority. By aiming to be a best estimate calculation or one that is
intentionally biased, the uncertainties in the conditions under which the code is applied is under the
control of the user.
The need for calculations to be best estimate necessitates that the code is developed and
validated against good quality data. The most obvious source of these is the power reactors for which
the calculations are to be applied. However, even if extensive Post Irradiation Examination (PIE) is
7
performed on discharged fuel, the lack of detailed information on the irradiation histories because of
little or no in-pile instrumentation means that the data are of limited value. Also, data obtained this way
takes a long time to obtain from the inception of the programme to the final examination. In-addition,
data are needed for fuel experiencing transients and other off-normal operating conditions which
cannot be reproduced under experimental conditions in a power reactor. For this reason the best
quality data for code development and validation comes from well instrumented and dedicated test
reactors.
There are many phenomena which are fundamental to all codes of which fuel temperatures and
fission gas release are the most obvious and must represent the core of the calculations. In other
areas, code requirements are country specific depending on the safety case that is supported. For
example, there are requirements in Germany, France and the U.K. to perform calculations on PCI
failure probability under transient over power conditions. Also, in the U.K., it is necessary to have
calculations on the inventory of radioactive iodine capable of immediate escape in the event of fuel
failure in coolant accidents. Thus, although the detailed requirements can be specific, there are many
aspects of fuel behaviour that are common, and for economic reasons, many companies have joined
International Projects and jointly funded experiments to obtain data.
8
CHAPTER 2
THE HALDEN REACTOR PROJECT
The OECD Halden Reactor Project is one such project. The Halden Boiling Water Reactor
(HBWR) currently operated at 18 to 20 MW is situated inconspicuously within the rock on the outskirts
of the town of Halden in southern Norway. The reactor has operated since 1959 and was initiated to
investigate nuclear capability as a supplier of steam to the wood pulp industry.
Since that time the reactor facilities have been updated progressively and through a series of
innovative techniques, the system has become one of the most versatile in the world. Over the course
of this development, some 300 in-pile experiments have been performed. These ranged in complexity
from rudimentary non-instrumented rod bundles to some of the most integrated and complex
in-reactor tests ever designed. The Project is conducted under the auspices of the OECD and is
sponsored financially through an agreement between the Institutt for Energiteknikk (IFE) of Norway
and a number of participating organisations throughout the world.
Conditions in the Halden reactor are particularly well suited to studies of fuel performance. The
boiling conditions ensure a constant coolant temperature, and hence a well defined boundary
condition from which to assess thermal performance from measurements of centreline fuel
temperatures. Also, the low system pressure of 32 bar and low fast flux ensures negligible clad creep
down, thus removing one parameter from through life assessments of fuel dimensions and
temperature. However, when more prototypic conditions are required, dedicated in-pile loops are
available to simulate the thermal hydraulic conditions of temperature and pressure as well as neutron
flux spectrum for PWR, BWR and most recently, Advanced Gas Cooled Reactors (CAGR) as
operated in the U.K.
This report outlines some of the experiments that have been performed in the Halden reactor and
the most important data that they have provided which are available for use in fuel performance
studies. As to be expected in a Project that has many years of experience and data production,
several reviews of individual topics have already been written. This present report has made extensive
use of these with the aim of demonstrating the extent of the information available collated in a form
that is most useful to code developers.
The Experimental Programme
The Halden experimental programme is conducted in two ways. The first is an agreed
programme of work over three year periods. This is called the Joint Programme, and all the data
generated are available to the Halden Project sponsors for their own use. Although the results are
proprietary, there has been a general rule that information is disclosed after a period of five years. The
second is a series of bi-lateral experiments performed by Halden staff specifically for member
organisations. These are funded entirely by the sponsor and the data are only disclosed to other
Project members at the sponsors discretion. When discussing the data available from The Project, the
degree of access will be indicated. Although this report is concerned only with fuel and materials
irradiated in the HBWR, it should be mentioned that The Project also carries out extensive Man
9
Machine Interaction studies on the same Joint Programme and bi-lateral bases for both the nuclear and
non-nuclear industry.
Each Halden reactor experiment is identified by an Instrumented Fuel Assembly number (IFA).
This refers to the rig design which can be re-used for a number of experiments by reloading with a
succession of different fuel rods. Thus the unique experiment identifier also includes a loading
number. IFA-562.2 for example refers to an assembly, IFA-562 which was first loaded as IFA-562.1 to
investigate the effect of pellet surface roughness on thermal performance, but was subsequently
reloaded with a completely different design of fuel rod to investigate the thermal response and fission
gas release at very high burnup.
The experiments performed either address single effects e.g., fuel temperatures, fission gas
release, dimensional changes, waterside corrosion etc., or, because of extensive in-pile
instrumentation, the combination of centreline thermocouples, pressure gauges and extensometers,
integral behaviour is also available. These experiments are particularly useful for code validation
because they test simultaneously code predictions of several parameters.
Each experiment is contained within a Zircaloy shroud thus isolating the assembly coolant and
thermal hydraulics from the reactor pool. Within the shroud there are: inlet and outlet turbines for flow
monitoring and inlet and outlet coolant thermocouples. These are used in conjunction with miniature
neutron detectors to accurately define the axial and radial power distribution of the fuel in the
assembly.
Fuel centreline temperatures are monitored using W3%Re/W25%Re thermocouples, although
other thermocouples are also employed depending on the temperature range to be measured. More
recently an expansion thermometer design has been developed. The advantage of this method is that
unlike thermocoupled rods with permanently installed cabling, these can be loaded and unloaded at
will. Differential transformers are extensively used in reactor for a number of applications to measure:
rod pressure, fuel and clad elongation and clad diameters. The use of He-3 coils or movable absorber
shields has made possible the power ramping of individual assemblies without changing appreciably
the integrated power of the reactor nor affecting the steady state operation of other experiments
irradiated concurrently. The periodic use of these features has enabled studies of power cycling and
load following regimes.
The in-pile instrumentation is continuously logged by a dedicated computer at 15 min intervals
throughout the whole of the reactor operating periods. Faster logging speeds can be instigated for
individual assemblies if special experiments are being performed. Also, in the event of a reactor
scram, the logging speed of pre-defined experiments is automatically increased to obtain kinetic data.
The range of experiments and their instrumentation to be included in this summary are given in
Table 1, taken from HWR-302, Reference [1].
In the main, the experiments have used fuel rods which, apart from their length which seldom
exceeds 1 m and more usually 500 mm or less, conform to typical BWR and PWR design. The usual
materials used are sintered UO pellets clad in Zr-2 unless otherwise stated. Material and design
2
variables studied extensively include: fuel density and grain size, hollow and solid pellets, fuel-to-clad
gap dimension, fill gas composition and pressure. Other studies have included comparisons with
sphere-pac, MOX and doped fuel as well as cladding with different compositions. In addition to the
development of dedicated loops, a further important development has been that of re-instrumentation
of fuel and cladding pre-irradiated outside of the Halden reactor. This has meant that high burnup
studies have been undertaken without having to wait for extended burnup accumulation.
More recently there has been the introduction of non-prototypic experiments designed to provide
specific materials properties data. In this category are pressurised tubes to provide information on clad
creep rates, and the isothermal irradiation of UO disks to provide information on fission gas diffusivity
2
and thermal conductivity as a function of burnup.
10
